A History of Science, vol 4 | Page 6

more than the original
product, and the natural inference from this would be that the metal
must have taken in some substance rather than have given off anything.
But the phlogistians had not learned the all-important significance of
weights, and their explanation of variation in weight was either that

such gain or loss was an unimportant "accident" at best, or that
phlogiston, being light, tended to lighten any substance containing it, so
that driving it out of the metal by calcination naturally left the residue
heavier.
At first the phlogiston theory seemed to explain in an indisputable way
all the known chemical phenomena. Gradually, however, as
experiments multiplied, it became evident that the plain theory as stated
by Stahl and his followers failed to explain satisfactorily certain
laboratory reactions. To meet these new conditions, certain
modifications were introduced from time to time, giving the theory a
flexibility that would allow it to cover all cases. But as the number of
inexplicable experiments continued to increase, and new modifications
to the theory became necessary, it was found that some of these
modifications were directly contradictory to others, and thus the simple
theory became too cumbersome from the number of its modifications.
Its supporters disagreed among themselves, first as to the explanation
of certain phenomena that did not seem to accord with the phlogistic
theory, and a little later as to the theory itself. But as yet there was no
satisfactory substitute for this theory, which, even if unsatisfactory,
seemed better than anything that had gone before or could be
suggested.
But the good effects of the era of experimental research, to which the
theory of Stahl had given such an impetus, were showing in the attitude
of the experimenters. The works of some of the older writers, such as
Boyle and Hooke, were again sought out in their dusty corners and
consulted, and their surmises as to the possible mixture of various gases
in the air were more carefully considered. Still the phlogiston theory
was firmly grounded in the minds of the philosophers, who can hardly
be censured for adhering to it, at least until some satisfactory substitute
was offered. The foundation for such a theory was finally laid, as we
shall see presently, by the work of Black, Priestley, Cavendish, and
Lavoisier, in the eighteenth century, but the phlogiston theory cannot
be said to have finally succumbed until the opening years of the
nineteenth century.

II. THE BEGINNINGS OF MODERN CHEMISTRY
THE "PNEUMATIC" CHEMISTS
Modern chemistry may be said to have its beginning with the work of
Stephen Hales (1677-1761), who early in the eighteenth century began
his important study of the elasticity of air. Departing from the point of
view of most of the scientists of the time, be considered air to be "a fine
elastic fluid, with particles of very different nature floating in it" ; and
he showed that these "particles" could be separated. He pointed out,
also, that various gases, or "airs," as he called them, were contained in
many solid substances. The importance of his work, however, lies in
the fact that his general studies were along lines leading away from the
accepted doctrines of the time, and that they gave the impetus to the
investigation of the properties of gases by such chemists as Black,
Priestley, Cavendish, and Lavoisier, whose specific discoveries are the
foundation-stones of modern chemistry.
JOSEPH BLACK
The careful studies of Hales were continued by his younger confrere,
Dr. Joseph Black (1728-1799), whose experiments in the weights of
gases and other chemicals were first steps in quantitative chemistry.
But even more important than his discoveries of chemical properties in
general was his discovery of the properties of carbonic-acid gas.
Black had been educated for the medical profession in the University of
Glasgow, being a friend and pupil of the famous Dr. William Cullen.
But his liking was for the chemical laboratory rather than for the
practice of medicine. Within three years after completing his medical
course, and when only twenty-three years of age, he made the
discovery of the properties of carbonic acid, which he called by the
name of "fixed air." After discovering this gas, Black made a long
series of experiments, by which he was able to show how widely it was
distributed throughout nature. Thus, in 1757, be discovered that the
bubbles given off in the process of brewing, where there was vegetable
fermentation, were composed of it. To prove this, he collected the
contents of these bubbles in a bottle containing lime-water. When this

bottle was shaken violently, so that the lime-water and the carbonic
acid became thoroughly mixed, an insoluble white powder was
precipitated from the solution, the carbonic acid having combined
chemically with the lime to form the insoluble calcium carbonate, or
chalk. This experiment suggested another. Fixing